Fan beam computed tomography apparatus

Abstract

A computed tomography has a frame carrying an x-ray source and an x-ray detector which are rotated by the frame around an examination subject to obtain a series of exposures from different angular positions. The x-ray source emits a fan beam from a focus and includes a focus deflecting unit for adjusting the position of the focus in the event that, due to mechanical wear or play, the frame is caused to rotate around a point which differs from the intended rotational center. The intended rotational center is a point through which a straight line extending between the focus and the center of the radiation detector always proceeds for every angular position. Two sensors are disposed on the rotating center frame in addition to the x-ray source and the radiation detector, these two sensors respectively defining two measuring points and supplying output signals corresponding to the respective positions of these measuring points relative to a reference point on a stationary part of the apparatus. The output signals are supplied to correction electronics, which also receives an output signal from a focus control unit, and effects a deflection of the focus by means of the focus deflecting unit as needed to maintain rotation of the frame around the intended rotational center.

We claim as our invention: 
     
       1. A computed tomography apparatus comprising: an x-ray source having a focus from which an x-ray fan beam emanates;   a rotating frame on which said x-ray source is mounted;   an x-ray detector mounted on said frame opposite said x-ray source, said rotating frame rotating said x-ray source and said detector around a rotational center;   means for deflecting a position of said focus;   a stationary apparatus part having a reference point thereon;   two sensors disposed on said frame and rotating with said frame around said rotational center, said sensors respectively defining two measuring points and generating output electrical signals corresponding to the position of said two measuring points with respect to said reference point on said stationary apparatus part;   a focus control unit; and   correction means, supplied with said output electrical signals from said two sensors and with an output signal from said focus control unit, for generating a control signal for operating said means for deflecting said focus for maintaining a straight connecting line extending between said focus and a center of said detector to always proceed through said rotational center.